The estrogen receptor (ER) plays a critical role in the development of the normal mammary gland and in more than two-thirds of human breast cancer where it is the most important therapeutic target. The central hypothesis addressed by this Project is that the growth promoting activity of estrogen in breast cancer and the efficacy of therapies that inhibit its receptor are the result of the abnormal functioning of components of the normal estrogen-signaling machinery. This project is based on recent progress in defining the complete set of ER cis-regulatory targets across the entire genome, which together constitute the "ER cistrome" and the identification of the critical roles played by the FoxA1 transcription factor and the CARM1 coregulator in estrogen-mediated signaling and growth of human ER+ breast cancer cells. In addition it builds on the description of connections between ER and the transcription factors FoxA1 and PDEF that characterize ER+ breast cancers. These studies will lead to a detailed understanding of the transcriptional network governing the development of the normal ER+ mammary lineage and the abnormalities in the network that support the growth of ER+ breast cancer. They will be significantly facilitated through important interactions with the Polyak, Sicinski, and Brugge Groups as well as external collaborations. Specifically in Aim 1 it will test whether Foxa1 plays an essential role in the development of the mammary gland;Aim 2 will determine the role of the coregulator CARM1 in AIB1-induced mammary hyperplasia and cancer;and Aim 3 will explore the role of the ets transcription factor PDEF in determining the phenotype of ER+ breast cancers and of the normal ER+ mammary epithelium.